Most people know that being exposed to the sun’s rays can be harmful (it can increase the risk of skin cancer), but they may not know there’s a risk of too little sun.

“Sun exposure is actually beneficial when obtained in small doses—such as 15 minutes or less several times a week. The sun’s ultraviolet rays actually trigger vitamin D synthesis in the skin,” says Nelson Watts, MD, director of the University of Cincinnati Bone and Osteoporosis Center. “Vitamin D is important because it helps us absorb calcium from the digestive system for our bones,” he says.

The days are getting shorter as winter approaches, and people are less exposed to the sun. Even if they are getting sun exposure, the winter sun is too weak to be beneficial.

According to Dr. Watts, a vitamin D deficiency prevents new bone tissue from hardening, a condition known as rickets in children and osteomalacia in adults. It can also result in bone loss and causes muscle weakness, which creates an increased tendency to fall.

Research studies have also found that insufficient levels of vitamin D may also contribute to developing different types of cancer such as breast, colon and prostate cancer, as well as heart disease and diabetes.

Here’s what you need to know about vitamin D:

What vitamin D is—Vitamin D is a fat-soluble vitamin found in some foods and which can be made in your body after exposure to ultraviolet rays from the sun.

Why vitamin D is important—Vitamin D helps the body maintain normal blood levels of calcium and phosphorus. It promotes calcium absorption and helps to form and maintain strong bones.

Who’s at risk for a vitamin D deficiency? Breastfeeding infants—Human breast milk alone doesn’t provide adequate vitamin D, and parents are advised to keep infants out of the sun. So the American Academy of Pediatrics recommends a daily supplement of 200 international units (IU) of vitamin D beginning in the first two months of life, unless they are weaned to receive at least 500 ml per day of vitamin D–fortified formula.

Older adults—The skin of adults aged 50 and older does not synthesize vitamin D efficiently and the kidney is less able to convert vitamin D to its active hormone form.

People with darker skin color—The melanin content is higher in people with darker skin, which reduces the skin’s ability to produce vitamin D from sunlight.

People with fat malabsorption—Because vitamin D is a fat-soluble vitamin, people with a reduced ability to absorb dietary fat, such as those with Crohn’s disease, cystic fibrosis, celiac disease or liver disease, or those who have had part or all of their stomach or intestines removed (as a result of gastric bypass surgery, for example), may not adequately absorb vitamin D.

People with limited sun exposure—Homebound individuals, people living in northern latitudes like New England and Alaska, and those who wear more protective clothing are at risk for vitamin D deficiency.

How much vitamin D do you need?—From birth to age 50, people should get 200 IU of vitamin D a day. Adults 51–69 require 400 IU daily, and those over 70 require 600 IU. Dr. Watts recommends patients with osteoporosis get 800–1,000 IU of vitamin D a day.

“The ‘safe upper limit’ for vitamin D is 2,000 IU per day,” says Dr. Watts, “and higher levels may be needed for some people. Although there is a wide margin of safety, taking too much vitamin D can lead to toxicity, which through high blood calcium levels can cause nausea, vomiting, poor appetite, constipation, weakness and weight loss. It can also cause heart rhythm abnormalities and confusion.”

Sources of vitamin D—Exposure to the sun provides most people with their vitamin D requirement, because ultraviolet rays trigger synthesis in the skin.

However, the sun’s rays are not strong enough in the winter and there are other ways to get vitamin D. There are some foods that contain vitamin D, such as egg yolks, sardines, mackerel, salmon, herring, fish liver oils, shrimp, chicken liver, oysters and fortified foods such as orange juice, milk and infant formula.

“It’s difficult to get your daily recommended amount from foods containing vitamin D,” says Dr. Watts. “Most multivitamins only contain 400 IU of vitamin D, so most people will need an additional supplement.”

Dr. Watts currently chairs the FDA’s Advisory Committee for Endocrine and Metabolic Drugs and is on the editorial boards of Osteoporosis International and the Journal of Clinical Densitometry. He is the past president of the International Society of Clinical Densitometry and past president of the American College of Endocrinology (ACE).

"UC Health Line” contains timely health information and is distributed every Tuesday by the UC Academic Health Center Public Relations and Communications Office.